SDI (Silt Density Index)

          An empirical test developed for membrane systems to measure the rate of fouling of a 0.45 micron filter pad by the suspended and colloidal particles in a feed water. This test involves the time required to filter a specified volume of feed at a constant 30 psi at time zero and then after 5 minutes, 10 minutes and 15 minutes of continuous filtration. Typical RO element warranties list a maximum SDI of 4.0 at 15 minutes for the feed water. If the SDI test is limited to only 5 or 10 minute readings due to plugging of the filter pad, the user can expect a high level of fouling for the RO. Deep wells typically have SDI’s of 3 or less and turbidities less than one with little or no pretreatment. Surface sources typically require pretreatment for removal of colloidal and suspended solids to achieve acceptable SDI and turbidity values.

Article by Sivanandan

T.P.Sivanandan

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